Aircraft: A321
Phase of flight: Initial Approach
Anomaly: ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy

I had been called out to replace the Captain who had called in sick for the last two legs of the trip. During the introduction with the First Officer; I learned it was to be his turn to fly the leg which was fine with me. I also learned he had been flying the Airbus for some time. Everything had gone very smoothly on the flight; the First Officer had made good decisions picking his way around thunderstorms all the way out west. We briefed the arrival as the Seavu Arrival to Runway 24R; which is what both of us had flown the last several times we had landed at LAX. I thought about putting 25L in the secondary flight plan at this time; but decided it was just as easy to change it on the primary should we be given that runway. As we began the arrival we were given a descent to FL240 and cleared direct to KONZL. Then turned out to the right with a vector to a heading; and then cleared back direct to KONZL. We were given a descent to 17;000 without the crossing restriction at KONZL. I assumed because they had already slowed us to 250 KTS and the clearance came a bit late. Seems like the next thing was we were turned over to SoCal Approach. When we checked in we were not given a runway or a descend via clearance; but given a direct to KRAIN clearance; which is on the 25L approach. As I was re-programming the computer for the 25L ILS we received the approach clearance for the 25L ILS; which I acknowledged with a cleared the ILS 25L; I missed the fix and altitude but figured we would be going right down the glideslope so it didn't really matter. At about this same time SoCal Approach requested us to slow from 250 KTS to 210 KTS which the First Officer started to do right away by extending the speedbrake. Unfortunately the FMS was still flying the descent profile for the arrival and so the aircraft didn't level off momentarily it just held to its profile even adding power to maintain it; which countered the speedbrake being out. SoCal then requested us to slow to 190 KTS; we still hadn't gotten to 240. I should mention here that it is a bit of a dance to get a max landing weight Airbus 321 to slow down and go down at the same time and the use of the speedbrake raises the Vls to above or equal to flap one extension speed. SoCal then requested slowing to 170 KTS; I acknowledged and put my hand near the landing gear lever saying we need to slow down; The First Officer agreed and asked for gear down. At that time we were about a half dot below the glideslope and the aircraft was still trying to fly the profile descent. The First Officer changed the FMS to a rate of descent that was too high. Then we not only had a speed problem but we went way below the glideslope. Keep in mind we are still 20 miles out from the airport we were never below the MSA for the area; just below the profile crossings and glideslope and it was VMC. I asked the First Officer why he wasn't capturing the glideslope and could not hear his reply. Then I requested that he level off so we could intercept the glideslope and continue the approach. About this time SoCal Approach asked us to level also and asked if we knew we were below the glideslope; I answered in the affirmative. I noticed we were at the 15 mile distance to the runway at about 2;500 FT indicated altitude so I gave the flight attendants the four bell signal and the rest of the approach and landing was normal. The event occurred due to several things. Though we were prepared and well briefed we did not receive the clearance expected; not the 'descend via' the Seavu arrival; not given a runway assignment when we checked in. It seemed like ATC was doing some training as we would hear different controller voices on the same frequency. when we were given the direct to KRAIN; which is on a different approach; I went to change the FMC but the First Officer had started the Direct to page and it (FMC) would not let me change the approach until he got out of the Direct to page which cost us a little time ata time we were a bit task saturated. But the thing that got us below the glideslope was; and I didn't understand this until well into our debrief; the First Officer thought he was above the glideslope; which is very common out there when ATC slows you way down on the approach; but we were full scale below it. And though it was obvious to me we were below it when I asked him to capture the glideslope that just made him descend more trying to catch where he thought it was. Fortunately we were quite a distance from the runway and we were at a safe altitude where we could level off; the First Officer could catch back up with the airplane and continue to an uneventful landing. Flying the line; one of the first things you learn is there is more that one way or technique to achieve the same results. Such as slowing a heavy Airbus down and going down at the same time. Many of the arrivals can be difficult for a max landing weight A321 to perform normally and when given a large de-acceleration on top of that adds to the difficulty. I have been critiqued during check rides; for my lack of assertiveness. I am mindful of that in these types of situations. I was monitoring what was happening and the safety of the flight while suggesting and requesting corrective actions for the First Officer to take; finally just saying; 'level off'. I suppose I should have said climb up and recapture the glideslope; that may have made him realize he was below it. (Which I didn't realize he thought he was above it until later while debriefing.) The First Officer admitted he had been critiqued for his soft quiet voice during training; and I had trouble hearing him especially with ATC in the other ear constantly transmitting in a busy area like L.A. Part of my pre-flight brief is; I spent ten years crop dusting; I'm half deaf in one ear and can't hear out of the other so please speak up. As far as corrections; I won't let anyone get that far off the glide path again before intervening. A321 Captain describes an LAX arrival that goes somewhat ary due to a late runway assignment and the First Officers lack of situational awareness.